Inspection method and inspection system using charged particle beam

ABSTRACT

Secondary electrons and back scattered electrons generated by irradiating a wafer to be inspected such as a semiconductor wafer with a charged particle beam are detected by a detector. A signal proportional to the number of detected electrons is generated, and an inspection image is formed on the basis of the signal. On the other hand, in consideration of a current value and irradiation energy of a charged particle beam, an electric field on the surface of the inspection wafer, emission efficiency of the secondary electrons and back scattered electrons, and the like, an electric resistance and an electric capacity are determined so as to coincide with those in the inspection image. In a state where a difference between a resistance value in a normal portion and a resistance value in a defective portion is sufficiently increased by using the charging generated by the irradiation of electron beams, an inspection is conducted to thereby detect a defect.

This Application is a Division of 09/785,275, filed Feb. 20, 2001, nowU.S. Pat. No. 6,618,850.

BACKGROUND OF THE INVENTION

The present invention relates to a method and system for manufacturing awafer having a fine circuit pattern such as a semiconductor device, orliquid crystal. More particularly, the invention relates to a techniqueof inspecting a pattern of a semiconductor device or a photomask, and toinspecting method and system using a charged particle beam, forinspecting a defect in an arbitrary part on an unfinished semiconductorwafer in a semiconductor device fabricating process.

A semiconductor device is manufactured by, for example, repeating a stepof transferring a pattern formed with a photomask on a semiconductorwafer by a lithography process and an etching process. In amanufacturing step of a semiconductor device, whether the lithographyprocess, etching process, and the like are successfully performed or notexerts a large influence on the yield of the semiconductor device. It isconsequently important to detect occurrence of an abnormal state or afailure at an early stage or before it occurs. Particularly, to improvethe yield, it is important to measure electric resistance and electriccapacity of a contact hole and an interconnection in a partiallyfinished semiconductor wafer at an early stage of the manufacturingprocess. There are the following conventional techniques of performinginspections to detect an electrical defect.

One of the techniques is a nano-probe device JP-A No. H8-160109) formeasuring electric resistance by making a sharpened tungsten (W) needle(with the tip having a radius of curvature of about 0.1 μm) come intodirect contact with a measurement portion. As patterns become finer inrecent years, however, the size of a portion to be measured becomesabout the same or smaller than that of the W needle, so that themeasurement is becoming very difficult. As means for dealing with theproblem, it can be considered to reduce the radius of curvature of thetip of the W needle. In this case, however, the tip becomes very softand is consequently deformed when it comes into contact with the portionto be measured. The method is not a realistic one. Another problem iscontact resistance. When the needle and the portion to be measured aremade of different materials, especially, one of them is made of asemiconductor, a Schottky junction occurs, and electric resistancedepending on a voltage occurs in the portion. Consequently, accuratemeasurement cannot be performed.

Another conventional technique uses a SEM (Scanning Electron Microscope)and is disclosed in JP-A No. H5-258703, JP-A No. H11-121561, and JP-ANo. H6-326165.

According to the technique disclosed in JP-A No. H5-258703, an image tobe inspected is compared with an adjacent image, and a portion havingdifferent potential contrast (brightness) is determined as a defect,thereby detecting a defect. The technique, however, does not have meansfor obtaining and displaying electric characteristics (electricresistance and electric capacity) and, therefore, cannot determinewhether the portion is critical or not.

According to the technique disclosed in JP-A No. H11-121561, the degreeof emission of secondary electrons is controlled by a control electrodepositioned on a wafer, the surface of the wafer is charged positively ornegatively, and a normal portion, a low-resistance defective portion,and a high-resistance defective portion are determined from a voltagecontrast image obtained at this time.

Control of the emission of secondary electrons by the control electrodeis disclosed in JP-A No. S59-155941. For example, in a voltage contrastimage obtained in the case where the control electrode is adjusted to bepositively charged, a low-resistance defective portion (for example, lowresistance of few hundreds Ω) is light, a high-resistance defectiveportion (electric resistance: ∞) is dark, and a normal portion is lightbut is darker than the low-resistance defective portion since the normalportion has resistance higher than the low-resistance defective portion.From the lightness/darkness of the image, the resistance can bedetermined to be high or low, but an absolute value cannot becalculated. By measuring a leak current, electric resistance can becalculated. However, it takes time for the inspection and the electricresistance cannot be measured at high speed.

According to the technique disclosed in JP-A-6-326165, aninterconnection on a substrate is irradiated with electron beams, adielectric current is generated between the substrate and the ground bycharges generated by the irradiation, and a change with time ismeasured. Although the electric capacity can be measured to be large orsmall, an absolute value cannot be measured.

In an inspection using such a voltage contrast image of the SEM, in somecases, a difference in potential contrast between the normal portion andthe defective portion is small depending on the structure of a wafer,and it is difficult to detect a defect. For example, in the case where acircuit has a pn junction, when the pn junction is reverse-biased bycharges which are generated in association with irradiation of electronbeams, the portion becomes high-resistant. Due to this, it is difficultto discriminate the portion from a high-resistant portion with faultyelectrical continuity.

As described above, since the nano-probe device has a problem such thata portion to be measured is smaller than the tip of the needle and aproblem of the contact resistance between the needle and a sample,accurate electric resistance cannot be measured depending on a sample.It takes very long time to inspect the whole face of a wafer and issubstantially impossible to conduct the inspection. Although anapparatus using the SEM can determine whether the electric resistance ishigh or low and whether the electric capacity is large or small from avoltage contrast image but cannot estimate an absolute value. Although aresistance value can be calculated by measuring a leak current, it takestime for an inspection, the inspection cannot be conducted at highspeed, and it takes very long time to inspect the whole face of a wafer.Further, depending on the structure of a wafer (for example, when a pnjunction is formed), it is difficult to discriminate a normal portionfrom a defective portion in a voltage contrast image.

SUMMARY OF THE INVENTION

An object of the invention is to provide an inspection method and systemfor automatically calculating values of electric resistance and electriccapacity and obtaining distributions and tendencies of electricresistance and electric capacity of the surface of a substrate such as awafer in short time only by acquiring a voltage contrast image. Anotherobject is to provide appropriate inspection parameters corresponding tothe structure of a wafer.

The inventors of the present invention have found that the objects canbe achieved by using the fact that a charged particle beam image(voltage contrast image) obtained by irradiating a sample with a chargedparticle beam depends on electric resistance and electric capacitybetween the irradiation region and the ground and irradiation time.

The mechanism will be described by referring to FIG. 2. Electrons areused as an example of particles applied. It is assumed that an incidentenergy EPE is about 500 eV where emission efficiency σ of the sum ofsecondary electrons and back scattered electrons is larger than 1.Usually, an electron having energy equal to or smaller than 50 eV iscalled a secondary electron and an electron having energy larger than 50eV is called a back scattering electron. When an insulator 60 isirradiated with an electron beam, an electron beam irradiated region 61is charged positively (a case where it is charged at 4V is shown as anexample) and a potential barrier of Us [eV] is formed on the surface.

Consequently, as shown by an energy ESE distribution of the sum NSE ofthe number of secondary electrons and back scattered electrons of FIG.3, the secondary electrons and back scattered electrons having energylower than Us are not emitted. Even when the secondary electrons andback scattered electrons are emitted, they are returned to the insulatorside. When the number of electrons returned is N₁, the number of emittedelectrons is N₂, and the ratio of the secondary electrons and backscattered electrons which are not returned but are emitted is σ_(SE),σ_(SE)=N2/(N₁+N₂). A substantial emission efficiency σ_(eff) isexpressed as σ_(eff)=σ_(SE)×σ. With the irradiation of electron beams,the charging voltage increases and the ratio of returned secondaryelectrons and back scattered electrons becomes higher. Consequently,σ_(eff) gradually decreases and, finally, stability is achieved with acharging voltage at which σ_(eff) is 1.

On the other hand, in the case of a conductor having sufficiently lowelectric resistance, electrons can be supplied. The charging istherefore lessened, and σ and σ_(eff) are almost equal to each other.The insulator looks dark and the conductor looks light.

An example by which the inventors of the present invention have foundthat electric resistance can be measured by using the above fact willnow be described. A sample used is a wafer as shown in FIG. 4 in whichan SiO₂ film 402 is formed on a silicon (Si) 407 and contact holes 401in each of which a tungsten (W) plug 400 (having a diameter of 0.25 μm)is buried are opened. Each plug has electric resistance due to a residue403 of SiO₂ (402) on the bottom of the plug. Reference numeral 404 showsa high-resistant open defect having the residue 403, and 405 denotes alow-resistant short defect in which plugs are connected.

An equivalent circuit shown in FIG. 5 including an electric resistor Rand an electric capacitor C of an electron carrying path was used. Fromthe shape of the plug, the electric capacitance C was set to 10⁻¹⁷ F(farad). Electron irradiation parameters in FIG. 23 were used. 10 keVwas set as the initial energy of an electron beam 19 emitted from anelectron gun 10, an earth voltage was applied to a retarding electrode63, and a retarding voltage of −9.5 kV was applied to a wafer 9 to beinspected. Consequently, the incident energy EPE of the electron beam tothe wafer was 500 eV.

As a result, it was found that the sum NSE of the number of secondaryelectrons and back scattered electrons to be detected changes with timeas shown in FIG. 6. The vertical axis denotes the value obtained bydividing NES to be detected by electron beam current IP [A], and thehorizontal axis indicates the product of electron beam irradiation timeTe (or time required for a scan electron beam to cross the plug) and IP.It was also found that the relation between NSE which was stabilizedafter elapse of sufficient time and the electric resistance R is asshown in FIG. 7. The vertical axis indicates NSE/IP and the horizontalaxis indicates the product of R and IP. In order to calculate theelectric resistance R, a change in NSE is necessary.

From the above, the inventors of the present invention have found forthe first time that the parameter of the current IP of the electron beamto calculate the electric resistance R is 0<log(R·IP)<3. Although thegraph of FIG. 7 depends on the shapes of material and sample, the changeamount is very small. By changing the retarding voltage V_(R) orcharging (voltage V_(B)) the peripheral region of a plug by applying anelectron beam before inspection, the NSE can be changed as shown inFIGS. 8 and 9, so that high-sensitive inspection parameters exist. Inorder to confirm the calculation prediction, each of plugs wasirradiated with an electron beam and the number of secondary electronsand back scattered electrons from the plug were detected.

After that, the electric resistance of each plug was measured by anano-prober. Since the uppermost layer of the plug is made of W, thecontact resistance with the W needle of the nano-prober is sufficientlylow. FIG. 10 shows the dependency on the electric resistance R of theNSE obtained as a result. The vertical axis indicates the quotient ofthe NSE and the beam current IP, and the horizontal axis denotes theproduct between the electric resistance R and the beam current Ip. Inthe graph, a solid line shows calculation and circles indicateexperiment. It was found that the calculation and the experimentcoincides with each other. That is, it was found that the resistancevalue can be calculated from the voltage contrast image by calculatingthe dependency on resistance of the sum of the number of secondaryelectrons and back scattered electrons.

It was also found that the electric capacity can be also measured byusing the semiconductor wafer. This will be described hereinbelow. Thebeam current IP was set to 100 nA. A plug having resistance 10¹² Ω (ohm)or higher recognized by the nano-probe method was used. By comparing theresult of calculation of a change in NSE to be detected which isobtained by changing irradiation time of the electron beam (or timerequired for the electron beam to cross the plug) with the calculationresult, the electric capacity was estimated from the comparison result.As shown in FIG. 11 of the experiment result and the calculation result,the change with time of the NSE when the electric capacity was set to10⁻¹⁷ F (farad) in the calculation result reproduces the experimentresult almost faithfully. The value is almost equal to the valuecalculated from the shape of the plug or the like.

Further, a method of providing appropriate inspection parameters adaptedto the structure of a wafer will now be described. FIG. 8 shows that theNSE changes by changing the retarding voltage V_(R). In this case, thecharging voltage US (V) in an electron beam irradiation region alsovaries as shown in FIG. 24. The phenomenon is used for the inspection.For example, when a circuit pattern having a pn junction is irradiatedwith an electron beam and the pn junction is charged so as to bereverse-biased, the junction has high resistance, and it is consequentlydifficult to discriminate the junction from a defect of faultyelectrical continuity. However, by changing the retarding voltage toincrease the charging voltage and to cause a breakdown in the pnjunction, the resistance value can be reduced. Since the differencebetween the resistance value in the normal portion and the resistancevalue in the defective portion becomes large and the normal portion andthe defective portion become different from each other in a voltagecontrast image, the inspection can be conducted in such a state.

In the case where the pn junction itself has a defect and the resistancevalue in the reverse bias direction is small, the retarding voltage isadjusted to a charging state where a breakdown is caused only in thedefective pn junction. The difference between the resistance value inthe normal portion and the resistance value in the defective portion isincreased, and a difference occurs between the normal and defectiveportions, so that the inspection can be conducted. Although theretarding voltage is used here, also by changing other electronirradiation parameters such as electron beam current, incident energy ofthe electron beam to a sample, irradiation time, and the number ofirradiation times, similar effects can be produced.

It was found that, by the methods as described above, the electricresistance and electric capacity can be estimated and the appropriatedefect detecting parameters adapted to the structure of a wafer can beprovided. Although the electron beam was used in the examples, thesimilar effects can be produced by using other charged particle beamssuch as positive ions and negative ions.

In an actual inspection system, the electric resistance and electriccapacity are determined automatically as follows.

First, a semiconductor wafer is scanned with a charged particle beamonce or a plurality of times. Scan time may be changed. Secondaryelectrons and back scattered electrons generated are detected by adetector, a signal proportional to the number of electrons detected isgenerated, and an inspection image corresponding to the scan is formedon the basis of the signal.

Subsequently, by a computer such as a work station or personal computer,in consideration of the current value of the charged particle beam,irradiation energy, scan time, the number of scanning times, electricfield on the surface of the semiconductor wafer, emission efficiency ofthe secondary electrons and back scattered electrons, and the like, animage is formed by using the electric resistance and electric capacityas parameters. The electric resistance and electric capacity aredetermined so that the image coincides with the inspection image.

In the inspection, by changing the current value of the charged particlebeam, the measurement range of the electric resistance can be changed.As a process before acquiring an image, the charged particle beam of apredetermined quantity is preliminarily applied or the retarding voltageis applied to the semiconductor wafer, thereby enabling measurementsensitivity to be changed.

Appropriate inspection parameters adapted to the structure of a wafercan be also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an inspection system according to anembodiment of the invention.

FIG. 2 is a diagram showing a state where the surface of a wafer ischarged and a potential barrier is formed in association withirradiation of electron beams.

FIG. 3 is a diagram showing an energy ESE distribution of a sum NSE ofthe number of secondary electrons and back scattered electrons.

FIG. 4 is a schematic diagram showing a cross-sectional structure of asemiconductor wafer.

FIG. 5 is a diagram showing an example of an equivalent circuit of acontact hole in FIG. 4.

FIG. 6 is a diagram showing a change with time in the sum NSE of thenumber of secondary electrons and back scattered electrons to bedetected.

FIG. 7 is a diagram showing the dependency on electric resistance R ofthe sum NSE of the number of secondary electrons and back scatteredelectrons to be detected.

FIG. 8 is a diagram showing the dependency on the electric resistance Rof the sum NSE of the number of secondary electrons and back scatteredelectrons to be detected when a retarding voltage V_(R) is changed.

FIG. 9 is a diagram showing dependency on the electric resistance R ofthe sum NSE of the number of secondary electrons and back scatteredelectrons to be detected when a charging voltage VB around a plug ischanged.

FIG. 10 is a diagram showing a comparison between calculation andexperiment of the dependency on the electric resistance R of the sum NSEof the number of secondary electrons and back scattered electrons to bedetected.

FIG. 11 is a diagram showing a comparison between calculation andexperiment of a change with time of the sum NSE of the number ofsecondary electrons and back scattered electrons to be detected.

FIG. 12 is a diagram showing a semiconductor wafer and a voltagecontrast image used in First Example.

FIG. 13 is a diagram showing potential contrast signals of thesemiconductor wafer in FIG. 12.

FIG. 14 is a diagram showing the potential contrast signal of FIG. 13and a calculated electric resistance value.

FIG. 15 is a diagram showing a change in time of irradiating beamshaving peak intensities indicated by the arrows in FIG. 13.

FIGS. 16(a) and 16(b) are diagrams showing inspection images 1 obtainedin Second Example.

FIG. 17 is a diagram showing an inspection image 2 obtained in SecondExample.

FIG. 18 is a diagram showing a sample holder with a standard resistancesample used in Third Example.

FIG. 19 is a diagram showing an inspection image obtained in FourthExample.

FIG. 20 is a diagram showing experiments and calculated values of achange with time in the sum of secondary electrons and back scatteredelectrons to be detected, which are obtained in Fifth Example.

FIG. 21 is a diagram showing an inspection image obtained in SixthExample.

FIG. 22 is a diagram showing an inspection image obtained in SeventhExample.

FIG. 23 is a diagram showing an example of electron beam irradiatingparameters.

FIG. 24 is a diagram showing dependency on resistance of a chargingvoltage in the case where a retarding voltage is changed.

FIG. 25 is a diagram showing a cross section of a wafer in which acontact hole having a pn junction is formed.

FIG. 26 is a diagram showing a voltage contrast image when a retardingvoltage is not proper.

FIG. 27 is a diagram showing the resistance of a pn junction as afunction of a voltage.

FIG. 28 is a diagram showing a voltage contrast image in the case wherethe retarding voltage is proper.

FIG. 29 is a diagram showing a voltage contrast image in the case wherethe retarding voltage is proper.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1 shows the configuration of an inspection system as a firstembodiment of the invention. The inspection system 1 has an inspectionchamber 2 evacuated to be in a vacuum, and a spare chamber (not shown inthis embodiment) for carrying a wafer 9 to be inspected into theinspection chamber 2. The spare chamber can be evacuated independent ofthe inspection chamber 2. The inspection system 1 has, other than theinspection chamber 2 and the spare chamber, a control unit 6 and animage processing unit 5. The inspection chamber 2 has therein, broadly,electron optics 3, a detector 7, a sample chamber 8, and an opticalmicroscope unit 4.

The electron optics 3 include an electron gun 10, an electron beamextraction electrode 11, a condenser lens 12, a blanking deflector 13, ascanning deflector 15, an aperture 14, an objective lens 16, a converterelectrode 17, and an EXB deflector 18. In the detector 7, a detector 20is disposed above the objective lens 16 in the inspection chamber 2. Anoutput signal of the detector 20 is amplified by a preamplifier 21installed on the outside of the inspection chamber 2 and is converted byan AD converter 22 into digital data. The sample chamber 8 includes asample holder 30, an x stage 31, a y stage 32, a rotation stage 33, astage position measurement unit 34,

-   -   and a wafer height measure sensor 35.

The optical microscope unit 4 is mounted in a position near butsufficiently apart from the electron optics 3 in the inspection chamber2 so as not to exert an influence on each other. The distance betweenthe electron optics 3 and the optical microscope unit 4 is known. The xstage 31 or y stage 32 reciprocates in the known distance between theelectron optics 3 and the optical microscope unit 4. The opticalmicroscope unit 4 has a white light source 40, a light lens 41, and aCCD camera 42. The image processing unit 5 includes an image memory unit46 and a calculator 48. An electron beam image or optical imageacquired, and measured electric resistance and electric capacity aredisplayed on a monitor 50.

Operation commands and operation parameters of the units in the systemare inputted/outputted from the control unit 6 The parameters such asacceleration voltage on generation of an electron beam, electron beamdeflection width, deflection speed, signal capturing timing of detector,and sample holder moving speed are preliminarily entered to the controlunit 6 so as to be optionally or selectively set according to thepurpose. The control unit 6 monitors a deviation in position and heightfrom signals of the stage position measurement unit 34 and the waferheight measure sensor 35 by using a correction control circuit 43,generates a correction signal on the basis of the monitoring result, andsends the correction signal to an objective lens power supply 45 and ascanning signal generator 44 so that the correct position is alwaysirradiated with the electron beam. To acquire an image of the surface ofthe inspection wafer 9, the inspection wafer 9 is irradiated with thenarrowed electron beam 19 to thereby generate secondary electrons andback scattered electrons 51, and the secondary electrons and backscattered electrons 51 are detected synchronously with the scanning ofthe electron beam 19 and the movement of the stages 31 and 32.

In the electron gun 10, a thermal electric field emitting electronsource of a diffusion supply type is used. By using the electron gun10,a stabler electron beam current as compared with conventionalelectron sources such as a tungsten (W) filament electron source and acold emission type electron source can be assured. Consequently, avoltage contrast image with little fluctuation in brightness can beobtained. The electron beam 19 is emitted from the electron gun 10 byapplying a voltage between the electron gun 10 and the extractionelectrode 11. The electron beam 19 is accelerated by applying a negativepotential of a high voltage to the electron gun 10.

The electron beam 19 travels toward the sample holder 30 with an energycorresponding to the potential, is condensed by the condenser lens 12,further narrowed by the objective lens 16, and is incident on theinspection wafer 9 (wafer having a fine circuit pattern, such as asemiconductor wafer, chip, liquid crystal, or mask) mounted on thex-stage 31 and the y-stage 32 on the sample holder 30.

A scanning signal generator 44 for generating a scanning signal and ablanking signal is connected to the blanking deflector 13, and theobjective lens power supply 45 is connected to each of the condenserlens 12 and the objective lens 16. A negative voltage (retardingvoltage) can be applied from a retarding power supply 36 to theinspection wafer 9. By adjusting the voltage of the retarding powersupply 36, a primary electron beam is decelerated, and an electron beamirradiation energy to the inspection wafer 9 can be adjusted to anoptimum value without changing the potential of the electron gun 10.

The secondary electrons and back scattered electrons 51 generated byirradiating the inspection wafer 9 with the electron beam 19 areaccelerated by the negative voltage applied to the wafer 9. The EXBdeflector 18 is disposed over the inspection wafer 9 and the acceleratedsecondary electrons and the back scattered electrons 51 are deflected bythe EXB deflector 18 to a predetermined direction. In accordance withthe voltage applied to the EXB deflector 18 and the intensity of themagnetic field, the deflection amount can be adjusted. Theelectromagnetic field can be varied interlockingly with the negativevoltage applied to the sample. The secondary electrons and backscattered electrons 51 deflected by the EXB deflector 18 collide withthe converter electrode 17 under predetermined conditions. When theaccelerated secondary electrons and back scattered electrons 51 collidewith the converter electrode 17, secondary electrons and back scatteredelectrons 52 are generated from the converter electrode 17.

The detector 7 has the detector 20 in the evacuated inspection chamber2, and the preamplifier 21, the AD converter 22, a converter 23, anoptical fiber 24, a converter 25, a high voltage power supply 26, apreamplifier power supply 27, an AD converter power supply 28, and apower supply 29 which are on the outside of the inspection chamber 2. Asdescribed above, the detector 20 in the detector 7 is disposed above theobjective lens 16 in the inspection chamber 2. The detector 20,preamplifier 21, AD converter 22, converter 23, preamplifier powersupply 27, and AD converter power supply 28 are floated at a positivepotential by the high voltage power supply 26. The secondary electronsand back scattered electrons 52 generated by the collision with theconverter electrode 17 are led to the detector 20 by a suction field.

The detector 20 is constructed to detect the secondary electrons andback scattered electrons 52 interlockingly with the timing of scanningthe electron beam 19, which are generated by the collision of thesecondary electrons and back scattered electrons 51 generated when theinspection wafer 9 is irradiated with the electron beams 19. An outputsignal of the detector 20 is amplified by the preamplifier 21 mounted onthe outside of the inspection chamber 2 and converted to digital data bythe AD converter 22. The AD converter 22 immediately converts the analogsignal detected by the detector 20 and amplified by the preamplifier 21and sends the digital signal to the image processing unit 5. Since thedetected analog signal is converted to a digital signal immediatelyafter the detection and the digital signal is transmitted, a signalhaving a high S/N ratio can be obtained at high speed. As the detector20, for example, a semiconductor detector may be used.

The inspection wafer 9 is mounted on the x and y stages 31 and 32. Atthe time of inspection, either a method of two-dimensionally scanningthe inspection wafer 9 with the electron beam 19 while the x and ystages 31 and 32 are kept still or a method of linearly scanning theinspection wafer 9 with the electron beam 19 in the x direction so thatthe x and y stages 31 and 32 are continuously moved at constant speed inthe y direction at the time of inspection can be selected. In the caseof inspecting a specific relatively small region, the former method ofcarrying out the inspection by keeping the stages still is effective. Inthe case of inspecting a relatively large region, the method of carryingout the inspection while continuously moving the stages at constantspeed is effective. When it is necessary to blank the electron beam 19,the electron beam 19 is deflected by the blanking deflector 13 so as notto pass through the aperture 14.

As the stage position measurement unit 34, a measurement unit usinglaser interference is employed in the embodiment. The positions of the xand y stages 31 and 32 can be monitored in a real-time manner and can betransferred to the control unit 6. Data such as rotational speeds of themotor for the x and y stages 31 and 32 and also the rotation stage 33 issimilarly transferred from their drivers to the control unit 6. Thecontrol unit 6 can accurately grasp the region and the positionirradiated with the electron beam 19 on the basis of the above data. Asnecessary, a positional deviation of the irradiation position of theelectron beam 19 is corrected in a real-time manner by the correctioncontrol unit 43. The region irradiated with the electron beam 19 can bestored every inspection wafer.

The wafer height measure sensor 35 takes the form of an optical measuresensor as a measure system which does not use an electron beam. Forexample, a laser interference measure sensor or a reflection typemeasure sensor for measuring a change in the position of reflectionlight is used. The wafer height measure sensor 35 measures the height ofthe inspection wafer 9 mounted on the x and y stages 31 and 32 in areal-time manner. In the embodiment, a method of irradiating theinspection wafer 9 with elongated white light which has passed through aslit through a transparent window, detecting the position of reflectionlight by the position detecting monitor, and calculating a change amountof the height from a fluctuation in position is used. On the basis ofthe measurement data of the wafer height measure sensor 35, the focaldistance of the objective lens 16 for narrowing the electron beam 19 isdynamically corrected and the electron beam 19 with the focal point in anon-inspection region can be always emitted. A warp and a distortion inheight of the inspection wafer 9 are preliminarily measured beforeirradiation of the electron beam. It is also possible to set correctionparameters of each inspection region of the objection lens 16 on thebasis of the measured data.

The image processing unit 5 is constructed by the image memory unit 46,calculator 48, and monitor 50. An image signal of the inspection wafer 9detected by the detector 20 is amplified by the preamplifier 21, andconverted by the AD converter 22 to a digital signal. The digital signalis converted by the converter 23 into a light signal. The light signalis transmitted via the optical fiber 24, and converted again to anelectric signal by the converter 25. The electric signal is stored inthe image memory unit 46.

The electron beam irradiation parameters for forming an image andvarious detection parameters of the detection system are preset at thetime of setting inspection parameters, and registered in files in adatabase. The calculator 48 reads the image signal supplied to the imagememory unit 46, calculates the correspondence between theabove-described image signal level and a surface charging voltage on thebasis of the electron beam irradiation parameters, and calculates theelectric resistance and electric capacity corresponding to the imagesignal level. The monitor 50 displays the electric resistance andelectric capacity calculated by the calculator 48 and/or the imagestored in the image memory unit 46.

An action in the case of measuring the electric resistance and electriccapacity of, for example, a partly-finished semiconductor wafer (havinga similar cross-sectional structure as that of FIG. 4) of 300 mmφ shownin FIG. 12 as the inspection wafer 9 by the inspection system 1 will nowbe described. First, although not shown in FIG. 1, a semiconductor waferis loaded by inspection wafer carrying means into a sample exchangechamber where the inspection wafer 9 is mounted on the sample holder andheld and fixed. After that, the sample exchange chamber is evacuated toa certain degree of vacuum. The inspection wafer 9 is transferred to theinspection chamber 2 for an inspection. In the inspection chamber 2, theinspection wafer 9 mounted on the sample holder is held and fixed on thesample holder 30, x and y stages 31 and 32, and rotation stage 33.

The set inspection semiconductor wafer 9 is disposed in predeterminedfirst coordinates below the optical microscope 4 by moving the x and ystages 31 and 32 in the X and Y directions on the basis of thepre-registered predetermined inspection parameters. An opticalmicroscope image of the circuit pattern formed on the inspection wafer 9is monitored by the monitor 50 and is compared with an equivalentcircuit pattern image in the same position prestored for positionrotation correction, thereby calculating a position correction value forthe first coordinates. The inspection wafer 9 is moved away from thefirst coordinates by a predetermined distance to second coordinateswhere a circuit pattern equivalent to that in the first coordinatesexists. Similarly, an optical microscope image is observed and iscompared with a circuit pattern image stored for position rotationcorrection, thereby calculating a position correction value of thesecond coordinates and a rotation deviation amount with respect to thefirst coordinates. The rotation stage 33 is rotated by the calculatedrotation deviation amount to correct its rotation amount.

Although the rotation deviation amount is corrected by rotating therotation stage 33 in the embodiment, it can be also corrected by amethod of correcting a scanning deflection position of the electron beamon the basis of the calculated rotation deviation amount without usingthe rotation stage 33. In the optical microscope image observation, acircuit pattern which can be observed by not only an optical microscopeimage but also an electron beam image is selected. For the positioncorrection in the future, the first coordinates, positional deviationamount of the first circuit pattern by the optical microscope imageobservation, second coordinates, and positional deviation amount of thesecond circuit pattern by the optical microscope image observation arestored and transferred to the control unit 6.

Further, an image observed by the optical microscope is used, thecircuit pattern formed on the inspection wafer 9 is observed, thepositions of chips and the distance between chips in the circuit patternon the inspection wafer 9, the pitch of repetitive patterns such asmemory cells, and the like are preliminarily measured, and measurementvalues are supplied to the control unit 6. A chip to be inspected on theinspection wafer 9 and a region to be inspected in the chip are set onthe basis of an image of the optical microscope and are supplied to thecontrol unit 6 in a manner similar to the above. An image of the opticalmicroscope can be observed at a relatively low magnification. When thesurface of the inspection wafer 9 is covered with, for example, asilicon oxide film, the underlayer can be also observed through thefilm. Consequently, the array of chips and the layout of circuitpatterns in a chip can be easily observed and the region to be inspectedcan be easily set.

After completion of the predetermined correcting work and the preparingwork such as setting of the region to be inspected by using the opticalmicroscope unit 4 as described above, by moving the x and y stages 31and 32, the inspection wafer 9 is moved under the electron optics 3.When the inspection wafer 9 is disposed under the electron optics 3,works similar to the correcting work and the setting of the inspectionregion performed by the optical microscope unit 4 are carried out byusing an electron beam image. In this case, the electron beam image isacquired as follows.

On the basis of the coordinate values stored and corrected in thepositioning using the optical microscope image, the same circuit patternas that observed by the optical microscope unit 4 is two-dimensionallyscanned in the XY directions and irradiated with the electron beam 19 bythe scanning deflector 44. By the two-dimensional scanning with theelectron beam, the secondary electrons and back scattered electrons 51generated from the portion to be observed are detected by theconfiguration and action of the above-described units for detectingelectrons, thereby obtaining an electron beam image. Since the simpleand easy inspection position recognition, positioning, positionadjustment, and also rotation correction have been carried out by usingthe optical microscope image, positioning, position correction, androtation correction can be performed with high accuracy at higherresolution and higher magnification as compared with the optical image.

When the inspection wafer 9 is irradiated with the electron beam 19, theirradiated position is charged. The charging sometimes distorts animage, so that it exerts an adverse influence on the inspection. Inorder to avoid the influence of the charging at the time of inspection,in a preparing work before the inspection such as the position rotationcorrection or inspection region setting, as a circuit pattern to beirradiated with the electron beam 19, it is arranged to automaticallyselect either a circuit pattern existing on the outside of theinspection region or an equivalent circuit pattern on a chip other thanthe inspection chip from the control unit 6. By the arrangement, theinfluence of the electron beam 19 emitted by the preparing work beforethe inspection is not exerted on the inspection image at the time ofinspection.

Subsequently, in an arbitrary region on the inspection wafer, electricresistance and electric capacity are measured. An image is obtained withincident energy of 500 eV and an electron beam current of 100 nA. FIG.13 shows signals of the secondary electrons and back scattered electronsgenerated between A and B of plugs 400 linearly arranged as shown inFIG. 12 in the obtained image. In graphs in FIG. 13, time Te ofapplication of beams on the plugs varies from 0.15 to 40 μs. Althoughsome peaks decrease with the electron irradiation time Te, stablesignals are obtained with sufficiently long Te (2.5 μs or longer in thiscase).

When the electric resistance was derived from a result of calculation asdescribed with reference to FIG. 7 under such circumstances, values(right vertical axis) in the range from 10⁷ to 10¹⁰ Ω (ohm) as shown inFIG. 14 were obtained. After that, the electric capacity was calculatedfrom the change in electron irradiation time Te at the peaks shown bythe arrows in FIG. 13. When the electric capacity was calculated fromthe relation between the intensity of the peak and Te shown in FIG. 15,a value of 10⁻¹⁷ F (farad) was obtained. By repeating the operation, theelectric resistance and electric capacity were measured with respect toall of selected inspection regions.

In the system, by changing the retarding voltage or preliminarilycharging an insulated portion to a certain degree by irradiation ofelectron beams before acquiring an image, a potential generated in frontof the plug is changed and, therefore, the sensitivity of resistancemeasurement can be changed as shown in FIGS. 8 and 9. It is understoodfrom FIG. 8 that, by decreasing the retarding voltage, the measurementrange can be narrowed. It is understood from FIG. 9 that, by chargingthe insulated portion, the measurement range can be narrowed. Althoughthe electron beam was used here as the charged particle beam, when anion beam is used instead of the electron beam, similar measurement canbe performed.

Second Embodiment

In a second embodiment, the electron beam image is acquired by using theinspection system described in the first embodiment and stored in theimage memory unit 46. Images of neighboring equivalent circuit patternsare compared with each other by the calculator 48. When a differentialsignal level is higher than a predetermined value, the portion in theelectron beam image is recognized as a defective portion and isdisplayed. Since the system has been described in detail in the firstembodiment, the description will not be repeated. From the image signallevel in the defective position, the electric resistance and capacityare calculated by the calculator 48. The degree of difference inresistance or capacity of the defective portion from the normal portionis obtained. There are the following three methods of obtaining an imageof the defective portion.

In a first method, an inspection of comparing the images with each otheris conducted. After completion of the inspection, an image is obtainedagain in the defect occurrence coordinates under the same parameters asthose in the inspection. The electric resistance and capacity arecalculated from the obtained image.

In a second method, in a manner similar to the first method, aftercompletion of the inspection, an image is obtained again in the defectoccurrence coordinates under electron beam parameters different fromthose in the inspection, and the electric resistance and capacity of thedefective portion are calculated from the correspondence of the imagesignal level with the image acquisition parameters. In the secondmethod, as the image acquisition parameters different from those for theinspection, the beam current, scan speed, the number of irradiationtimes, or the like is changed. An image may be acquired while changingone of the parameters or a plurality of parameters to calculate theelectric resistance and capacity.

In a third method, an image determined as defective in an inspection isautomatically stored, an image signal in the defective portion is read,and calculation is executed. In this case, it is unnecessary to obtainan image again in the defective portion after completion of theinspection.

An example of the inspection method of carrying out the inspection inthe second embodiment and displaying the electric defect level of thedefective portion will be described. As an example, FIG. 16(a) shows acircuit pattern including a defective portion after carrying out theinspection of a semiconductor wafer having 300 mmφ, and FIG. 16(b) showsan equivalent circuit pattern adjacent to the circuit pattern of FIG.16(a). When arbitrary defects in the circuit pattern are specified fromthe screen, the image of defective portions is displayed as shown inFIG. 17 and, simultaneously, the electric resistance R and the electriccapacity C of each of the defective portions are displayed (a defectiveportion (1) of 10¹⁰ Ω and 10⁻¹⁶ F and a defective portion (2) of 10⁸ Ωand 10⁻¹⁷ F). As references, the electric resistance (10⁷ Ω) and theelectric capacity (10⁻¹⁷ F) of a normal portion can be similarlydisplayed.

The kind of each of the defective portions can be determined from theelectric resistance R and the electric capacity C For example, since theresistance R and capacity C in the defective portion (1) are higher thanthe references, it is determined that the defective portion (1) iscompletely non-conductive due to a residual film. Since the resistance Ris higher than the reference resistance while no change occurs in thecapacity, it is determined that the portion (2) is defective due todeficiency of the plug material buried in the contact hole.

Third Embodiment

A third embodiment relates to an inspection method and inspection systemfor detecting, as a defect, a circuit pattern having resistancedifferent from a predetermined resistance, which is derived from thebrightness of an image by obtaining an electron beam image, andsimultaneously comparing the brightness of the image in the circuitpattern position with predetermined brightness by using the inspectionsystem described in the first embodiment.

First, in FIG. 18, a wafer 181 to be inspected is loaded into theinspection system, and pre-registered electron beam irradiationparameters and signal detection parameters are set. On a sample holder180, a standard sample 182 of which resistance under predeterminedelectron beam parameters is preliminarily calculated is attached. Beforestarting the inspection, an image of the standard sample 182 isacquired, and the correspondence between the resistance and capacity andthe electron beam image signal level is calibrated.

On the basis of a design value of the resistance or capacity of theinspection wafer 181, a predetermined permissible level range isdetermined and set. After that, an electron beam image of an arbitraryregion in the inspection wafer 181 is obtained. An image brightnesssignal obtained at the time of the inspection is compared with the setpermissible level. A circuit pattern having brightness out of thepermissible level is recognized and detected as a defect.

Fourth Embodiment

A fourth embodiment relates to an inspection method and inspectionsystem for calibrating the correspondence between the electron beamimage brightness and the electric resistance and capacity by the methoddescribed in the third embodiment by using the system described in thefirst embodiment, acquiring an electron beam image of a set arbitraryregion, and changing colors according to the levels of resistance ordisplaying the levels of resistance by contour lines. FIG. 19 shows anexample of an inspection result. By changing colors according to theresistance levels or displaying contour lines, a distribution of theresistance absolute levels is derived.

Fifth Embodiment

A fifth embodiment relates to an example of irradiating a predeterminedcircuit pattern with an electron beam under the condition that thepattern is not scanned with an electron beam by using the systemdescribed in the first embodiment and measuring the electric resistanceand electric capacity from a change in the signal level of the circuitpattern. In the embodiment, by measuring a change with time in the sumNSE of the number of secondary electrons and back scattered electronsemitted from an arbitrary region in the inspection wafer 9 and detected,the electric resistance and the electric capacity are estimated. Forthis purpose, first, as a pre-process, a region around the arbitraryregion is scanned with the narrowed electron beams 19 and a voltagecontrast image is obtained.

After that, from the image, a deflection parameter for irradiating thearbitrary region with an electron beam is set, the electron beam isapplied with the parameter, and a change with time in the sum NSE of thenumber of secondary electrons and back scattered electrons detected isobtained. FIG. 20 shows the result obtained at this time. The valuematches well with the result of calculation using the electricresistance of 10⁸ Ω and the electric capacity of 10⁻¹⁷ F. In such amanner, the electric resistance and electric capacity can be determined.

Sixth Embodiment

A sixth embodiment relates to a method and system for inspecting, forexample, a residue 214 of a resist 231 on a semiconductor wafer 212 byusing the system described in the first embodiment. According to theinspection method and system, the correspondence between the electronbeam image brightness and the electric resistance and capacity iscalibrated by the method described in the third embodiment, an electronbeam image of a set arbitrary region is acquired, and the residue isdisplayed in place of resistance level. FIG. 21 shows an inspectionimage 210, and an expected cross section image 211 of a wafer between Aand B in the inspection image 210. In accordance with the degrees of thereside 214, colors may be changed or contour lines can be displayed.

Seventh Embodiment

A seventh embodiment relates to a method and system for inspecting thewhole face of a semi-finish semiconductor wafer having 300 mmφ on whicha circuit is formed by using the system described in the firstembodiment. FIG. 22 shows the result of the inspection on the wholesurface of the wafer in a few hours by the method described in thesecond embodiment. A portion having resistance of 10⁸ Ω or higher isdisplayed as a defect 220.

Eighth Embodiment

An example of providing appropriate inspection parameters in the casewhere an electric resistance value of a pattern has dependency onvoltage in the circuit pattern defect inspection (for example, a casewhere a pn junction portion or a Schottky junction portion is formed)will be described.

FIG. 25 is a cross section showing an example of contact holes eachhaving a pn junction (502 denotes a p-Si and 502 indicates an n-Si). Aninspection of poor conduction of such a contact hole was conducted byusing an electron beam (IP=100 nA) of incident energy of 500 eV. FIG. 26shows a voltage contrast image when the retarding voltage is 0V. Thepotential contrast of the contact hole is too low to discriminate adefective portion.

In order to clarify the cause, resistance on voltage of a normal contacthole and that of a contact hole with a residue were measured. The leftgraph in FIG. 27 shows the result. 503 indicates the normal contact holeand 504 indicates the contact hole with a residue. In the graph, aresistance value of the normal contact hole 503 is low in the case of aforward bias (negative voltage) and is high in the case of a reversebias (positive voltage). It is understood that, a breakdown occurs when4V or higher is applied even in the case of the reverse bias, and theresistance value decreases. In this case, since the resistance value ofthe contact hole 504 having the residue is high, the difference betweenthe resistance value of the normal portion and that of the defectiveportion is sufficiently large.

On the other hand, in the first embodiment, when the retarding voltage(or electric field near the wafer) is changed, dependency on resistanceof the NSE changes as shown in FIG. 8, and it is understood that theresistance value measurement sensitivity can be changed. At this time,the charging voltage in the electron beam irradiated region changes aswell. FIG. 24 shows the dependency on resistance of the charging voltagein the case where the retarding voltage V_(R) is changed. An incidentenergy is 500 eV, the total electron emission efficiency is higher than1, and the surface of a wafer is positively charged. It is understoodthat the charging voltage increases due to decrease in the retardingvoltage V_(R) (from 0 to −28.5 kV) The charging voltage in the casewhere the retarding voltage is 0V is about 3V at the maximum.Specifically, since the resistance value of the normal contact hole issufficiently high, no leak current is occurs, the surface of the waferis charged to a voltage about 3V, and the resistance value becomes about10¹⁸ Ω. At this time, as understood from the right graph in FIG. 27, thenumber of emitted electrons is small, the voltage contrast image isdark, and the portion cannot be determined as a poor conduction portion.It is therefore considered that the voltage contrast image as shown inFIG. 26 is obtained as a result.

To enable an inspection to be conducted, a method of decreasing theresistance value of a pn junction by generating the charging voltagewhich is high enough to cause a breakdown in the pn junction is used. Bysetting the retarding voltage to −28.5 kV, the maximum charging voltagebecomes 5V or higher (FIG. 24), it is expected that a breakdown occursin the pn junction and the resistance value is sufficiently reduced (10⁸ Ω or lower). Consequently, a difference occurs between the resistancevalue of the normal portion and the resistance value of the poorconduction portion. It is assumed from the right graph of FIG. 27 thatthe difference between their voltage contrast images (NSE) issufficiently large, so that an inspection can be conducted. Practically,the retarding voltage was set to −28.5 kV and the voltage contrast image(FIG. 28) was obtained. As a result, a sufficient difference occursbetween the voltage contrast image of the normal portion and that of thedefective portion, and the defective portion can be displayed as a“defect” as in the second embodiment. In such a manner, the electronbeam irradiation parameters can be determined from the dependency onresistance of the charging voltage and the electric characteristics ofthe circuit.

A wafer having a defect in a pn junction (without a residue) can be alsoinspected. An inspection example in the case where a breakdown occurswith a low charging voltage due to a defect such that the concentrationof an n-diffusion layer in the pn junction is insufficient and aresistance value is consequently small will now be described. Asindicated by 505 in FIG. 27, the resistance value is smaller than thatof a normal portion. In this case, the inspection can be conducted withthe retarding voltage of 0V. Since no breakdown occurs in the normal pnjunction, the resistance value is sufficiently high and the voltagecontrast image is dark. On the other hand, a breakdown occurs in thedefective pn junction, the resistance is small, and the voltage contrastimage is light. The obtained voltage contrast image is as shown in FIG.29. The inspection parameters can be determined in such a manner.

Although the retarding voltage is changed in the above case, the purposeis to change the electric field around the wafer. Therefore, in place ofchanging the retarding voltage, by changing the position of an oppositeelectrode or peripheral electrode to change the electric field aroundthe wafer, equivalent effects can be obtained.

Although the case where the retarding voltage is used as a parameter hasbeen examined here, with respect to the other electron beam irradiationparameters (such as beam current, beam energy, and irradiation time) aswell, optimum parameters depending on the characteristics of a sampleexist.

Although an electron beam is used as a charged particle beam in theembodiments described above in detail, similar measurement can be madeby using an ion beam in place of the electron beam.

As described above, according to the inspection method and system of theinvention, in an inspection of a wafer partially completed such as asemiconductor device having a circuit pattern, the electric resistanceand electric capacity of a small region which cannot be measured by aconventional inspection system can be measured. By applying theinvention to the wafer manufacturing process, the electric resistanceand electric capacity can be estimated. Consequently, a process ofdealing with a failure can be performed immediately in the wafermanufacturing process. As a result, a fraction defective ofsemiconductor devices and other wafers is reduced and the productivitycan be increased.

Since occurrence of an abnormal state can be immediately detected fromthe measurement, occurrence of a number of defects can be prevented.Further, as a result, the occurrence of defects can be reduced. Thus,the reliability of a semiconductor device or the like can be increased,an efficiency of development of a new product improves, and themanufacturing cost can be reduced.

1. An inspection method of inspecting a pattern region by irradiating apattern region formed in the surface of a wafer with a charged particlebeam, comprising the steps of: irradiating said pattern region with saidcharged particle beam so as to be charged; generating a differencebetween an electric resistance value in a normal portion and an electricresistance value in a defective portion in said pattern region;detecting signals of secondary electrons or back scattered electronsgenerated from said region which is irradiated with said chargedparticle beam; and acquiring a voltage contrast image of said regionfrom a change in an emission efficiency of said secondary electrons orback scattered electrons in said normal portion and said defectiveportion.
 2. An inspection method using a charged particle beam accordingto claim 1, further comprising: analyzing said voltage contrast image ofsaid region; and determining an electric resistance and electriccapacity in said region.
 3. An inspection method using a chargedparticle beam according to claim 1, wherein said irradiating step causesa breakdown at a pn junction which is formed in said pattern region. 4.An inspection method of inspecting a pattern region by irradiating apattern region formed in the surface of a wafer with a charged particlebeam, an electric resistance value of said pattern region havingdependency on voltage, comprising the steps of: irradiating said patternregion with a charged particle beam so as to be charged; generating adifference between an electric resistance value in a normal portion andan electric resistance value in a defective portion in said patternregion on the basis of the relations between a voltage in the chargedstate and charged particle beam irradiating parameters including acharged particle beam current, incident energy of a charged particlebeam to said wafer, irradiation time, and a number of irradiation times;detecting signals of secondary electrons or back scattered electronsgenerated from said region which is irradiated with said chargedparticle beam; and acquiring a voltage contrast image of said regionfrom a change in an emission efficiency of said secondary electrons orback scattered electrons in said normal portion and said defectiveportion.
 5. An inspection method using a charged particle beam accordingto claim 4, further comprising: analyzing said voltage contrast image ofsaid region; and determining an electric resistance and electriccapacity in said region.
 6. An inspection method using a chargedparticle beam according to claim 4, wherein said irradiating step causesa breakdown at a pn junction which is formed in said pattern region.